Radio frequency amplifiers are used to amplify received communication signals before they are demodulated and decoded. Example signals include L-band modulated MPEG-2 signals broadcast from a satellite service. The signals may be demodulated and decoded in an integrated receiver/decoder (IRD) such as a television set top box. The IRD can be connected to a distributed infrastructure to allow multiple users to view the information encoded in the signal, or to single television set to view video content encoded in the signal. A simple amplifier may be used, such as START420 amplifier manufactured by ST Microelectronics. The START420 amplifier is a simple generic bipolar junction transistor (BJT) device designed for amplifying radio frequency (RF) signals. Such amplifiers exhibit a gain tilt of about 4.5 dB from 950 MHz to 2.5 GHz, and the input return loss is very poor. In order to use such an amplifier in a direct broadcast satellite (DBS) system, the bias network, input and output matching networks are modified. However, the gain tilt and input return loss are still worse than specified for some systems.
The input return loss can be improved by using a balanced amplifier design that has two identical amplifiers connected between quadrature hybrids, such as Lange couplers or branchline couplers. However, this increases the power draw of the amplifier and does not improve the gain tilt.